Process for manufacturing electronic circuits

ABSTRACT

The process for manufacturing an electronic circuit includes disposing an electronic device on a circuit substrate and hot melting a solder formed on the electronic device or the circuit substrate to bond the electronic device and the circuit substrate. The process includes the steps of feeding a liquid onto lands on the circuit substrate, aligning and mounting the electronic device on the lands, placing the circuit substrate in a treating vessel and heating the circuit substrate. The heating step includes controlling a pressure of an atmosphere in the treating vessel, hot-melting the solder to prevent at least a portion of the liquid from evaporating until the electronic device and the circuit substrate are bonded and to permit the liquid to evaporate after the electronic device and the circuit substrate are bonded.

This is a divisional application of U.S. Ser. No. 09/704,784, filed Nov.3, 2000 now U.S. Pat. No. 6,269,998, which is a divisional applicationof Ser. No. 09/538,515, filed Mar. 30, 2000 now U.S. Pat. No. 6,161,748,which is a divisional application of U.S. Ser. No. 09/322,998, filedJun. 1, 1999 now U.S. Pat. No. 6,133,135, which is a divisionalapplication of U.S. Ser. No. 08/647,672, filed May 15, 1996 now U.S.Pat. No. 5,940,728.

BACKGROUND OF THE INVENTION

This invention relates to a process for manufacturing an electroniccircuits wherein an electronic device or component (part) such assemiconductor integrated circuit (LSI) is connected to a circuitsubstrate, especially by soldering without using flux.

Soldering between a circuit substrate and a semiconductor integratedcircuit (LSI) or the like has required that the surfaces of the metalsin question to be joined should be kept clean and free from any materialhindering wettability.

Plating has also required that the surface of the metal in question tobe plated should be kept clean so that oxide film or layer or the likemay not exist thereon.

When an Au wire or Au ribbon or the like is to be bonded on the surfaceof the metal in question by ultrasonic heat pressure technique, oxidefilm on the surface of the metal in question is also troublesome andtherefore the surface of the metal in question must be kept clean.

The material hindering wettability of solder includes oxides, chlorides,sulfides, carbonates, various types of organic compounds, etc.Particularly, the most serious obstacle in the processes such assoldering, plating or ultrasonic heat pressure bonding of an Au wire orAu ribbon is oxide film present on the surface of the metal in questionsuch as solder, nickel (Ni), nickel alloys (alloys of nickel with othersubstances).

This oxyde film is usually chemically dissolved by flux and convertedinto a liquid compound. Thus, the surface of the metal in question andmetal atoms of solder get an opportunity to directly collide with eachother to form a metallic bonding state sharing an outer electron shell,so that the metal in question and solder can be alloyed. However, fluxresidues still remain on the surface and must be cleaned off.

The presence of oxide film hinders plating. For example, oxide film actsas an insulating (dielectric) film to prevent electric conductiontherethrough necessary for electroplating which is a typical example ofplating and thus hinders such electroplating.

Oxide film also hinders replacement plating by preventing the reactionbetween the surface of the metal in question and a plating liquid.

In these plating processes, oxide film should be also removed by liquidtreatment with hydrochloric acid or the like. However, residues stillremain and are largely responsible for lowering the bonding reliability.Residues have been conventionally cleaned off with chlorofluorocarbons.

Recently, a technology has been developed to omit the step of cleaningoff flux residues by using abietic acid (rosin) which leaves less amountof residues as flux and a minor amount of adipic acid or the like.However, the bonding reliability is insufficient.

This technology is described in detail in “Alumit Technical Journal 19”(1992) and “Action Mechanism and Problems of Cleaning-Free Flux” byKubota of Japan Industrial Technology Development Research Institute,Co.

A glazing method has also been proposed, which provides highlycorrosion- and abrasion-resistant materials with very fine homogeneoustexture or amorphous structure by irradiating metallic materials, steelmaterials, carbides or the like with laser beam. This glazing method isused for processing metallic materials to be subjected to hightemperatures and high pressures such as materials for automobileturbines, and discussed in, for example, “Laser Processing, SecondSeries”, by A. Kobayashi, pp. 164, published by Kaihatsusha.

A known method for removing oxide film on the surfaces of metals withoutusing flux or hydrochloric acid employs argon sputtering.

Japanese Patent Application Laying Open (KOKAI or UnexaminedPublication) No. 63-97382 discloses a method for providing a highlyadhesive pinhole-free coating film by plating a surface of, a metallicworkpiece, roughened by blasting with an alloy element and thereaftermelt processing the plated layer by irradiation with laser beam.

Further, Japanese Patent Application Laying Open (KOKAI) 62-256961discloses a method for preparing a surface-treated layer with goodcorrosion resistance and solderability by forming an anodized layer onthe surface of a base material formed of aluminium or alloys thereof.

SUMMARY OF THE INVENTION

As a result of reviews and studies of the above prior arts, theinventors have found the following problems.

(1) The method for removing oxide film by means of flux before solderinga circuit substrate and an integrated circuit involves the problem ofnecessarily requiring the step of cleaning off flux residues. Acid orthe like remaining as residues may also cause the corrosion of metal.

In addition, a drying step is indispensable after cleaning.

(2) The method for removing oxide film by argon sputtering involves theproblem of requiring a treatment in vacuum, with the result that theequipments can hardly be controlled and electronic devices or activeelements of electronic devices may be unfavorably affected by argonsputter.

(3) The laser beam-madiated glazing method and the laser processingmethod disclosed in Japanese Patent Application Laying Open (KOKAI)63-97382 will disadvantageously cause oxide film to grow duringsolidification of the metal surface because the both methods force themetal texture on the surface to melt by high energy laser beam in orderto have abrasion resistance or denseness on the metal surface.

The surface treatment method disclosed in Japanese Patent ApplicationLaying Open (KOKAI) 62-256961 can not be applied because it does notbelong to the technique for removing oxide film.

The object of this invention is to solve the above problems of the priorarts and provide a process for manufacturing electronic circuits,according to which soldering can be carried out without using flux byapplying a metal surface treatment procedure which allows oxide film,organic matters, carbon or the like on the surfaces of metals to beeasily removed without complex process nor unfavorably affectingelectronic devices, components or circuit substrates.

According to this invention, the above object is attained by a procedureof connecting an electronic device or component and a circuit substrateby means of solder, comprising the steps of irradiating said solder withlaser beam to clean said solder, aligning or registering, and mountingsaid electronic device on said circuit substrate, and hot-melting saidsolder in a low-oxygen content atmosphere to bond said electronic deviceand said circuit substrate with each other.

In this procedure, the surface of a metal such as solder is irradiatedwith laser beam having a lower energy than the energy changing thetexture of the surface of the metal. Thus, only the bonds between metalatoms and oxygen atoms on the surface of the metal are broken orreleased by the energy of the laser beam to remove oxide film as well asorganic matters, carbon, etc. on the surface while the metal texture onthe surface remains unmolten.

Oxide film on the surface of the metal can successfully be removedwhether said laser beam irradiation takes place in any of the atmospheresuch as air or He gas, or in vacuum.

Then, the electronic device or component and circuit substrate to beconnected by solder are aligned or registered by a pre-setting liquid,and thereafter they are soldered together by hot-melting the solder fromwhich has been removed oxide film in a low-oxygen content atmosphere.Thus, good soldering can be achieved without oxidizing solderedsurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view for illustrating a metal surface treatmentprocedure according to Example 1 of this invention.

FIG. 2 is a sectional view for illustrating a variation or modificationof Example 1 wherein the surfaces of solder bumps on a semiconductorintegrated circuit (LSI) or the like are irradiated with laser beam viaa lens or mirror, instead of the solder layer of FIG. 1.

FIG. 3 is a scanning electron micrograph of the surface of the solderlayer before irradiation with laser beam according to the Example 1.

FIG. 4 is an enlarged photograph of FIG. 3.

FIG. 5 is a scanning electron micrograph of the surface of the solderlayer after irradiated with laser beam according to the Example 1.

FIG. 6 is an enlarged photograph of FIG. 5.

FIG. 7 is a graph plotting the relation between the oxide film level orproportion (%) on the Sn—Pb surface as ordinate and the laser beamirradiation energy density per pulse (J/cm²) as abscissa in Example 1.

FIG. 8 is a graph plotting the oxide film level (%) on the Sn—Pb surfaceas ordinate and the irradiation cycle number (number of irradiations) ata fixed energy density of 1.5 (J/cm²) as abscissa in Example 1.

FIG. 9 is a sectional view showing an example of an electronic circuitsoldered according to Example 1.

FIG. 10 is a sectional view showing another example of an electroniccircuit soldered according to Example 1.

FIG. 11 is a sectional view for illustrating a metal surface treatmentprocedure according to Example 2 of this invention.

FIG. 12 is a graph plotting the relation between the thickness of theoxide film (nm) formed on the nickel layer as ordinate and the laserbeam irradiation energy density (J/cm²) as abscissa when the laser beamirradiation cycle number for the same zone of the nickel layer is fixedin Example 2.

FIG. 13 is a graph plotting the relation between the thickness of theoxide film (nm) formed on the surface of the nickel layer as ordinateand the laser beam irradiation cycle number for the same zone of thenickel layer as abscissa when the laser beam irradiation energy densityis fixed in Example 2.

FIG. 14 is a sectional view for illustrating a process for making anelectronic device according to Example 3 of this invention.

FIG. 15 is a configurative sectional view of an electronic device towhich the anti-reoxidizing means has actually been applied in Example 3.

FIG. 16 is a sectional view showing an example wherein an electronicdevice and a nickel (Ni) layer or nickel alloy layer on a ceramicsubstrate are directly electrically connected by solder without using aninput/output (I/O) pin of FIG. 15.

FIG. 17A is a plan view for illustrating a process for making anelectronic circuit according to Example 4 of this invention.

FIG. 17B is a sectional view taken along a line XVIIB—XVIIB of FIG. 17A.

FIG. 18 is a sectional view showing the configuration of a circuitsubstrate on which has been pre-set an electronic device to be solderedby a manufacturing apparatus according to an embodiment of thisinvention.

FIG. 19 is a perspective view showing the configuration of an apparatusfor manufacturing electronic circuits according to an embodiment of thisinvention.

FIG. 20 is a sectional view showing an inside of a treating vessel.

FIG. 21 lists examples of the liquid used for pre-setting an electronicdevice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of this invention will bow be described indetail by way of example with reference to the accompanying drawings.

EXAMPLE 1

FIG. 1 is a sectional view for illustrating a metal surface treatmentprocedure or process or method according to Example 1 of this invention,which shows a ceramic substrate 1, a metallized layer 2, a solder layer3 a, oxide film or layer 4, laser beam 5, a lens 6, and a mirror 7.

According to the metal surface treatment procedure of the embodiment 1,the oxide 4 grown (or residues of organic matters, carbon, etc.) on thesurface of the solder layer 3 a on the surface of the metallized layer 2formed on the top of the ceramic substrate 1 is removed as shown in FIG.1.

The metallized layer 2 is made of, for example, a titanium (Ti), nickel(Ni), or nickel alloy film.

In order to remove the oxide 4 (or residues of organic matters, carbon,etc.) on the surface of the solder layer 3 a, the surface of the solderlayer 3 a is irradiated with the laser beam 5 via the lens 6 and themirror 7. The oxide 4 is thus removed.

FIG. 2 shows a variation or modification of Example 1, wherein thesurfaces of solder bumps 3 b on a semiconductor integrated circuit (LSI)or the like are irradiated with laser beam 5 via a lens 6 and a mirror7, instead of the solder layer 3 a of FIG. 1.

The laser beam 5 used in Example 1 has a lower energy than the energyrequired to change the metallic texture or structure of the solder layer3 a or solder bumps 3 b. More specifically, it has an energy which ishigher than the bonding energy between Sn (tin) atom and O (oxygen) atomon the surface of the solder layer 3 a or solder bumps 3 b but lowerthan the bonding energy between Sn—Pb atoms.

When the solder layer 3 a or solder bumps 3 b is irradiated with suchlaser beam 5, only the bonds of Sn—Pb atoms with O atoms on the surfaceare broken or released by the energy of the laser beam 5 while thesolder on the surface remains unmolten. The oxide film 4 on the surfaceof the solder layer 3 a or solder bumps 3 b is thus removed. Organicmatters, carbon, etc. on the metal surface are also removedsimultaneously.

Since the main purpose of the irradiation with the laser beam 5 hereinis to break or release the bonds of Sn—Pb atoms with O atoms on thesurface, the laser beam 5 is preferably pulsed laser beam with a pulsewidth of 1 μs or less, for example.

Assuming that the bonds between Sn—Pb atoms and O atoms on the surfaceare broken by pulsed laser beam with a pulse width of 1 μs or less, thelaser beam 5 is preferably excimer laser beam with short wavelength(high photon energy), for example.

The oxide film 4 on the surface of the solder layer 3 a or solder bumps3 b could be conveniently removed whether the irradiation with the laserbeam 5 took place in any of the gaseous atmosphere such as air or He gasor in vacuum.

FIG. 3 is a photograph of the surface state or condition of the solderlayer 3 a observed by a scanning electron microscope before irradiationwith laser beam, and FIG. 4 is an enlarged photograph of FIG. 3.

These photographs show black residues of organic matters or carbon, etc.on the surface of the solder layer 3 a.

FIG. 5 is a photograph of the surface state or condition of the solderlayer 3 a similarly observed by a scanning electron microscope afterirradiation with laser beam, and FIG. 6 is an enlarged photograph ofFIG. 5.

These photographs show that residues of organic matters, carbon, etc.have been substantially wholly removed.

FIG. 7 is a graph plotting the relation between the oxide film level orproportion (%) on the Sn—Pb surface as ordinate and the laser beamirradiation energy density per pulse (J/cm²) (laser beam irradiationenergy per unit area) as abscissae.

FIG. 7 shows that the residual oxide film level is lower than theuntreated oxide film level when the laser beam irradiation energydensity ranges from 0.5 J/cm² to 4.0 J/cm², and especially reaches theminimum when the laser beam irradiation energy density is 1.5 J/cm².

The oxide film level herein is the oxygen content measured by energydiffusion X-ray spectroscopy (EDX).

FIG. 8 is a graph plotting the oxide film level or proportion (%) on theSn—Pb surface as ordinate and the laser beam irradiation cycle number(i.e. number of times of irradiation) as abscissa when the laser beamirradiation energy density is fixed at 1.5 (J/cm²).

FIG. 8 shows that the residual oxide film level is low when theirradiation cycle number ranges from 6 to 10, and especially reaches theminimum when the irradiation cycle number is 8.

These results prove that the oxide film level on the Sn—Pb surfacereaches the minimum and the wettability of the solder layer 3 a orsolder bumps 3 b is improved in a case where 8 cycles of laser beamirradiation are applied at an energy density of 1.5 J/cm².

FIG. 9 shows a cross section of a main part of an electronic circuitmade by flux-free soldering an integrated circuit (LSI) 8 onto ametallized layer 2 formed on the top of a ceramic substrate 1 afterremoving oxide film on the surface of solder bumps 3 b according to themetal surface treatment procedure of Example 1, and FIG. 10 shows across section of a main part of an electronic circuit similarly made byflux-free soldering after removing oxide film on the surface of solderbumps 3 b under a sealing cap 9.

EXAMPLE 2

FIG. 11 is a sectional view for illustrating a metal surface treatmentprocedure or process or method according to Example 2 of this invention.

According to the metal surface treatment procedure of Example 2, anoxide 4 (or residues of organic matters, carbon, etc.) on the surface ofa nickel (Ni) layer (or nickel alloy layer) 2 a formed on the top of aceramic substrate 1 is removed as shown in FIG. 11.

The nickel (Ni) layer (or nickel alloy layer) 2 a is normally liable tobe oxidized so that the oxide film 4 is readily formed on the surface ofthe nickel (Ni) layer or nickel alloy layer 2 a.

In order to remove the oxide 4 on the surface of the nickel layer 2 a,the surface of the nickel layer 2 a is irradiated with laser beam 5 viathe lens 6 and the mirror 7, in the same way as the above Example 1.

FIG. 12 is a graph plotting, by way of example, the relation between thethickness (nm) of the oxide film 4 formed on the nickel 2 a as ordinateand the laser beam irradiation energy density (J/cm²) (laser beamirradiation energy per unit area) as abscissa when the cycle number ofirradiation with the laser beam 5 for the same zone of the nickel layer2 a is fixed at 10.

FIG. 12 shows that a larger amount of oxide film can be removed as theirradiation energy density of the laser beam 5 increases, and that theoxide film 4 can be similarly removed even when the initial thickness ofthe oxide film is changed.

FIG. 13 is a graph plotting, by way of example, the relation between thethickness of the oxide film 4 (nm) formed on the surface of the nickellayer 2 a as ordinate and the laser beam irradiation cycle number forthe same zone or area of the nickel layer 2 a as abscissa when theirradiation energy density of the laser beam 5 is fixed at 0.75 (J/cm²).

FIG. 13 shows that the thickness of the oxide film decreases as theirradiation cycle number increases.

EXAMPLE 3

FIG. 14 is a sectional view for illustrating a process for making anelectronic device such as semiconductor integrated circuit (LSI)according to Example 3 of this invention.

In Example 3, an oxide (or residues of organic matters, carbon, etc.) onthe surface of a nickel (Ni) layer (or nickel alloy layer) 2 a formed onthe top of a ceramic substrate 1 is removed by the metal surfacetreatment procedure of the above Example 1 or 2, and then a platinglayer 10 is applied, as shown in FIG. 14.

The plating may be any of electroplating, electroless plating orreplacement plating, but the plating material herein is generally gold(Au) to prevent reoxidation.

The oxide film on the nickel (Ni) layer or nickel alloy layer 2 a ofmetallized layer can thus be removed while preventing the layer fromreoxidation.

FIG. 15 shows a configurative sectional view of an electronic device towhich the anti-reoxidizing means of Example 3 has actually been applied.

In the process of Example 3, a nickel (Ni) layer (or nickel alloy layer)2 a of metallized layer is formed on a ceramic substrate 1 and anorganic insulating layer 15 is applied thereon. This organic insulatinglayer 15 is perforated to expose said nickel (Ni) layer 2 a and an oxidefilm on the surface of thus exposed nickel (Ni) layer 2 a is removed bythe surface treatment procedure of the above Example 1 or 2, andthereafter an anti-reoxidizing plating layer 10 is applied. Then, aninput/output (I/O) pin 12 is bonded by solder 11.

As a result of applying the anti-reoxiding plating layer 10 afterremoving the oxide on the surface of the nickel alloy layer 2 a, a goodelectric connection can be achieved between the input/output (I/O) pin12 of a semiconductor integrated circuit (LSI) or the like and theceramic substrate 1.

The input/output (I/O) pin 12 and the nickel (Ni) layer 2 a on theceramic substrate 1 can be electrically connected in good conditions bythe solder 11 without applying the anti-reoxidizing plating layer (Auplating) 10 within about one week after the oxide film 4 has beenremoved by laser beam 5.

FIG. 16 shows a case where an electronic device and a nickel (Ni) layer2 a on a ceramic substrate 1 are directly electrically connected bysolder 11 without using an inlet/outlet (I/O) pin 12 shown in FIG. 15.

Conventionally, such a connection always required fluxes, but theprocess of Example 3 eliminates the necessity of fluxes.

EXAMPLE 4

FIGS. 17A and 17B illustrate a process for making an electronic devicesuch as semiconductor integrated circuit according to Example 4 of thisinvention, wherein FIG. 17A is a plan view and FIG. 17B is a sectionalview taken along the line XVIIB—XVIIB of FIG. 17A.

In the process of Example 4, a metal film 13 (for example, chromium(Cr), titanium (Ti)) with good adhesive properties to an organicinsulating or dielectric layer 15 is formed on the organic insulatinglayer 15 and a nickel (Ni) layer (or nickel alloy layer) 2 a is appliedthereon, as shown in FIGS. 17A and 17B. Oxide (or resudues of organicmatters, carbon, etc.) on the surface of this nickel (Ni) layer (ornickel alloy layer) 2 a is removed by the surface treatment procedure ofthe above Example 1 or 2, and thereafter a gold (Au) ribbon or gold (Au)wire 14 is bonded by ultrasonic heat pressure technique.

Such bonding has been usually difficult on the nickel (Ni) layer ornickel alloy layer 2 a due to the presence of oxide film on the surface,but good bonding can be achieved as a result of removing the oxide bythe procedure of the above Example 1 or 2.

Although the foregoing Examples include the metal surface treatment ofthe solder layer 3 a or nickel (Ni) layer 2 a, this invention is notlimited to these Examples but may be applied to various metals fromwhich oxide film or organic matters should be removed.

It is needless to say that the laser beam energy may be suitablycontrolled depending on the nature or property of the metal material.

Although pulsed laser beam was mentioned above as an example, similareffects can be achieved by continuous irradiation with laser beam havinga longer wavelength such as CO lasers provided that appropriatecontrolling means is added to prevent the metal texture itself frommelting.

The laser irradiation may cause the metal texture on the surface tomelt, but it is permissible so far as it lasts briefly.

EXAMPLE 5

FIG. 18 is a sectional view showing a configuration or structure of acircuit substrate on which has been pre-set an electronic device to besoldered by a manufacturing apparatus according to an embodiment of thisinvention, FIG. 19 is a perspective view showing a configuration orstructure of an apparatus for manufacturing electronic circuitsaccording to an embodiment of this invention, FIG. 20 is a sectionalview showing an inside or inner structure of a treating vessel, and FIG.21 lists examples of the liquid used for pre-setting an electronicdevice.

FIGS. 18 to 20 show an electronic device 21, a liquid 22 for pre-settingthe electronic device 21, lands for electric connection 23, a circuitsubstrate 24, solder 25, a treating vessel 26, a pressure controllingsection 27, an oxygen content monitoring section 28, a temperaturecontrolling section 29, an electronic circuit substrate transferringsection 30, a controlling section 31, an electronic circuit substrate tobe treated 32, a carbon heater 33, a cooling plate 34, a gas introducingsystem 35, and a vacuum exhaust or evacuation system 36.

As shown in FIG. 18, the electronic circuit substrate structure to betreated 32 formed of an electronic device and a circuit substrate whichare to be soldered together according to the embodiment of thisinvention comprises the electronic device 21 such as LSI provided withsolder bump terminals of solder 25 and pre-set on the circuit substrate24 made from ceramic, glass or epoxy, etc. by the liquid 22 forpre-setting the electronic device 21 thereon. The electronic device 21and the circuit substrate 24 are aligned or registered so that each ofthe lands 23 on the circuit substrate 20 to be soldered and thecorresponding solder 25 on the electronic device 21 may be matched eachother.

As shown in FIG. 19, an apparatus for manufacturing electronic circuitswherein the electronic circuit substrate to be treated 32 formed of theelectronic device 21 pre-set on the circuit substrate 24 is treated tobond solder 25 on the electronic device 21 with the lands 23 on thecircuit substrate 24 according to the embodiment of this invention asdescribed above comprises the treating vessel 26 for carrying outsoldering by heating, cooling or otherwise treating the electroniccircuit substrate to be treated 32 as shown in FIG. 18, the pressurecontrolling section 27 for controlling the evaporation rate of theliquid 22, the oxygen content monitoring section 28 for monitoring theoxygen content in a low-oxygen content or concentration atmosphereformed in the treating vessel 26, the temperature controlling section 29for the carbon heater 33 heating the electronic circuit substrate to betreated 32, the electronic circuit substrate transferring section 30 forautomating a series of transfer operations and the controlling section31 for automatically controlling the whole apparatus.

As shown in FIG. 20, the treating vessel 26 contains therein the carbonheater 33 for heating an electronic circuit substrate to be treated 32,and the water-cooled metallic cooling plate 34 for cooling the heatedcarbon heater 33 and electronic circuit substrate to be treated 32. Theelectronic circuit substrate to be treated 32 is disposed and treated onthe carbon heater 33.

The gas introducing system 35 and the vacuum exhaust or evacuationsystem 36 are connected to the treatment vessel 26 to control thetreatment atmosphere inside the treating vessel 26.

The treating vessel 26, gas introducing system 35, vacuum exhaust system36 and heating means such as the carbon heater 33 constitute a reflowheating system.

Now, a soldering process using an apparatus for manufacturing electroniccircuits according to the embodiment of this invention is describedbelow.

At first, a precontrolled amount of the liquid 22 is fed onto thecircuit substrate 24 by a dispenser (not shown). This feed amount iscontrolled to be capable of covering the solders 25 and the lands 23 butnot to move up the electronic device 21 even by the surface tension ofthe liquid 22 or other action. Then, the electronic device 21 such asLSI is mounted on the circuit substrate 24 so that the solder 25provided beforehand on the circuit substrate 24 may be aligned orregistered with the corresponding or respective lands 23 on theelectronic device 21 coated with the liquid 22.

The circuit substrate 24 carrying the electronic device 21 forms anelectronic circuit substrate to be treated 32 as shown in FIG. 18, whichis then placed in the electronic circuit substrate transferring section30 of the manufacturing apparatus of FIG. 19. The electronic circuitsubstrate to be treated 32 placed in the electronic circuit substratetransferring section 30 is transferred by a sort of robot, such asprogram-controlled manipulator or transfer mechanism, onto the carbonheater 33 in the treating vessel 26, as shown in FIG. 20.

Then, the gas in the treating vessel 26 is exhausted or evacuated by avacuum exhaust system 36 formed of a rotary pump or the like and anon-oxidizing gas such as He, nitrogen (N) or a reducing gas such asmixture of hydrogen (H) and nitrogen (N) is introduced via a flow- andpressure-controllable gas introducing system 35 to once restore theinside of the treating vessel 26 to atmospheric pressure. If the oxygencontent (concentration) within the treating vessel 26 as measured by theoxygen content monitoring section 28 is not lowered to a predeterminedlevel (preferably 10 ppm or less), the above vacuum exhaustion and gasintroducing steps are repeated until the predetermined low-oxygencontent atmosphere is formed. The low-oxygen content atmosphere has aneffect of inhibiting the oxidation of the circuit substrate 24, lands ofthe electronic device 21 and solder 25 on the electronic circuitsubstrate to be treated 32 during heating.

After the low-oxygen content atmosphere has been formed in the treatingvessel 26, the electronic circuit substrate to be treated 32 is heatedby direct thermal conduction from the carbon heater 33 while theoccurrence or abnormalities is constantly monitored during heating. Thisheating is controlled at a temperature higher than the melting point ofthe solder 25 by the temperature controlling section 29. When themelting point of the solder 25 is 221° C., for example, the temperatureof the carbon heater 33 is set at 250° C.

When the heating starts, the liquid 22 used for pre-setting theelectronic device 21 on the circuit substrate 24 begins to evaporate. Ifit is desired to promote or suppress this evaporation, the pressure ofthe gas introduced to form a low-oxygen content atmosphere as describedabove may have been set lower or higher than atmospheric pressure. Afterthe solder 25 melts and soldering is completed, cooling water issupplied to the water-cooled metallic cooling plate 34 to cool theheated carbon heater 33 and electronic circuit substrate to be treated32, and then the electronic circuit substrate to be treated 32 is takenout by the substrate transferring section 30.

Now, a process for soldering the electronic device 21 to the circuitsubstrate 24 is specifically explained, which enables to omit thecleaning step after soldering by allowing the liquid 22 on theelectronic circuit substrate to be treated 32 to completely evaporate byusing the manufacturing apparatus according to the embodiment of thisinvention.

In one embodiment of this invention, a solder having a melting point of220° C. is used and the temperature of the carbon heater 33 is set at250° C.

As the liquid for pre-setting the electronic device 21 on the circuitsubstrate 24, a rosin-free alcoholic liquid as shown in FIG. 21 may beused, for example.

The liquid A shown in FIG. 21 is ethylene glycol having a boiling pointof 197° C. which is relatively lower than the melting point 221° C. ofthe solder 25 used. Such a liquid begins to evaporate as the temperatureof the electronic circuit substrate to be treated 32 rises after thecarbon heater 33 starts to heat it. In this embodiment which uses thesolder having a melting point of 221° C. and the carbon heater 33 risingup to 250° C., the liquid will have completely evaporated beforesoldering is completed. Once the liquid has completely evaporated andthe solder 25 has been completely molten to complete soldering, coolingwater is supplied to the cooling plate 34 to cool the heated carbonheater 33 and electronic circuit substrate to be treated 32 and then theelectronic circuit substrate to be treated 32 is removed or taken out bythe substrate transferring section 30.

If the electronic device 21 has been pre-set on the circuit substrate 24by the liquid having the boiling point relatively lower than the meltingpoint of the solder 25 used as described above, the soldering can becarried out without leaving any trace of the liquid used for pre-settingand therefore the cleaning step after soldering can be omitted accordingto this embodiment.

On the contrary, the liquid B shown in FIG. 21 is triethylene glycolhaving a boiling point of 287° C., which is relatively higher than themelting point 221° C. of the solder 25 used. Such liquid slowlyevaporates even if the temperature of the electronic circuit substrateto be treated 32 rises after the carbon heater 33 starts to heat it, andthe liquid will not completely evaporate and will be left aftersoldering is completed.

In order to solve this problem, the treating vessel 26 is evacuated tolower its inner pressure after soldering is completed. This allows theliquid to have completely evaporated before the electronic circuitsubstrate to be treated 32 is cooled by the cooling plate 34, andenables to omit the cleaning step after soldering similarly to the casewhich uses the liquid A.

The evaporation rate of the liquid A or B is regulated by controllingthe pressure in the treating vessel 26 so that the liquid which haspre-set the electronic device may be left until the solder melts duringsoldering but the liquid may have completely evaporated when solderingis completed by the molten solder. Since the liquid is left until thesolder melts, solder joints can be protected against oxidation and morereliable soldering can be carried out even in an atmosphere containingsome level of oxygen.

As apparent from the foregoing description, this invention allows oxidefilm on the surface of metal to be removed without using flux byirradiating the surface of the metal such as solder with laser beamhaving a lower energy than the energy changing the texture of thesurface of the metal.

This invention also prevents the oxidation of circuit substrates, landsor contact pads of electronic devices and solder during heating andensures more reliable soldering by pre-setting an electronic device anda circuit substrate by a specific liquid and hot-melting solder in alow-oxygen content atmosphere.

Finally, this invention enables to omit the cleaning process using fluxsuch as chlorofluorocarbons, thus enables to prevent harmful influenceson the global environment as well as to reduce production equipments andproduction steps.

What is claimed is:
 1. A process for manufacturing an electronic circuitincluding disposing an electronic device on a circuit substrate, andhot-melting a solder material formed on said electronic device or saidcircuit substrate to bond said electronic device and said circuitsubstrate, said process comprising the steps of: irradiating a laserbeam on said solder material to remove an oxide film on a surface ofsaid solder material; feeding a liquid onto a connecting surface on saidcircuit substrate; aligning and mounting said electronic device on saidconnecting surface; disposing said circuit substrate in a treatingvessel; heating said circuit substrate to hot-melt said solder material;controlling a pressure of an atmosphere in said treating vessel toprevent at least a portion of said liquid from evaporating until saidelectronic device and said circuit substrate are bonded; and making saidliquid evaporate after said electronic device and said circuit substratehave been bonded.
 2. The process according to claim 1, wherein said stepof controlling said pressure of said atmosphere comprises the step oflowering the pressure of said atmosphere to make said liquid evaporateafter said electronic device and said circuit substrate have beenbonded.
 3. The process according to claim 1, wherein said step ofdisposing said circuit substrate in said treating vessel furthercomprises the step of forming an optional oxygen content atmosphere insaid treating vessel.
 4. The process according to claim 1, wherein saidstep of disposing said circuit substrate in said treating vessel furthercomprises the step of forming an atmosphere of an oxygen concentrationbelow 10 ppm in said treating vessel.
 5. The process according to claim4, wherein said atmosphere forming step comprises the step ofdischarging and introducing a gas while monitoring said oxygenconcentration.
 6. The process according to claim 5, wherein said gasintroducing step comprises the step of introducing a non-oxidizing orreducing gas.
 7. The process according to claim 1, wherein said liquidsupplying step comprises the step of supplying a rosin-free alcoholicsolution as said liquid.
 8. The process according to claim 7, whereinsaid liquid supplying step comprises the step of supplying ethyleneglycol or triethylene glycol as said alcoholic solution.
 9. The processaccording to claim 1, wherein said laser beam irradiating step comprisesthe step of irradiating a laser beam having an energy density of 0.5 to4.0 J/cm².
 10. The process according to claim 1, wherein said laser beamirradiating step comprises the step of irradiating a laser beam having apulse width of 1 μs (microsecond).
 11. The process according to claim 1,further comprising the step of cooling said circuit substrate after saidelectronic device and said circuit substrate have been bonded.
 12. Aprocess for manufacturing an electronic circuit including disposing aLSI, which is formed with a plurality of solder bumps, on a circuitsubstrate, and hot-melting said plurality of solder bumps to bond saidLSI and said circuit substrate, said process comprising the steps of:irradiating an excimer laser beam on said plurality of solder bumps toremove an oxide film on surfaces of the respective solder bumps;supplying an alcoholic solution to a connecting surface on said circuitsubstrate; aligning and mounting said LSI on said connecting surface,disposing said circuit substrate in a treating vessel; forming anatmosphere of a predetermined oxygen concentration in said treatingvessel; heating said circuit substrate to hot-melt said plurality ofsolder bumps; controlling a pressure of said atmosphere in said treatingvessel to prevent at least a portion of said alcoholic solution fromevaporating until said LSI and said circuit substrate are bonded; andmaking said solution evaporate after said LSI and said circuit substratehave been bonded.